README: add badges

6 jobs for v3.0.2 in 1 minute and 18 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #86278
doc-coverage

00:00:43

48.0%
passed #86279
format

00:00:14

passed #86274
robotpkg-example-adder-18.04-release

00:01:18

passed #86277
robotpkg-py-example-adder-py2-18.04-release

00:00:38

passed #86275
robotpkg-py-example-adder-py3-16.04-release

00:00:35

passed #86276
robotpkg-py-example-adder-py3-18.04-release

00:00:39